At microwave frequencies there is a range of antenna devices that can be used. These include slotted waveguide arrays, printed patch arrays, and reflector and lens systems. Above about 20 GHz slotted waveguide arrays require high tolerances and are thus expensive to manufacture in large quantities. For example at 20 GHz a large slotted waveguide array may need around 2000 slots, each of which must be individually machined to precise dimensions.
The aperture coupled patch array has all of the active elements of the antenna, radiating elements, transmission lines, coupled slots etc., on different layers of PCB. The elements are placed on the PCB using the conventional techniques of photo-lithography. In order for the device to work the layers must be very carefully lined up and must be carefully spaced apart. The tolerance limit for alignment and spacing between the layers is very tight and thus large arrays are difficult to mass produce.
Printed patch array antennae suffer from inferior efficiency due to high dissipative losses of transmission lines, particularly at high frequencies and for large arrays. In order to avoid radiation losses from the lines it is necessary to keep the spacings within the order of 0.01 .lambda.. Furthermore the restrictions on spacing mean that the transmission lines must be very thin. As they are thin they will have high losses and thus be inefficient for large arrays. Frequency bandwidths for such antennae are typically less than that which can be realized with slotted planar arrays, that is to say they are particularly narrow.
Reflector and lens arrays are generally employed in applications for which the additional bulk and weight of a reflector or lens system are deemed to be acceptable. The absence of discrete aperture excitation control in traditional reflector and lens antennae limit their effectiveness in low sidelobe and shaped beam applications.
Increasingly, as such antennae are becoming more widespread, and concern for the quality of the environment is growing, the use of lens or reflector systems is becoming less and less publicly acceptable. It is therefore desirable to provide a flat plate antenna array having the advantages of a lens or reflector but without the environmental impact.